PATHCHAT Edition No. 87.2
Published: May 2024
Please contact your local Ampath pathologist for more information.
Author:
- Dr. Muller Rossouw
Key Messages
✅ Organophosphates and Carbamates are Cholinesterase Inhibitors
- These chemicals disrupt nervous system function by inhibiting cholinesterase enzymes.
- Commonly used in pesticides, posing occupational risks to exposed workers.
✅ Cholinesterase Monitoring is Legally Required for Workers Exposed to Organophosphates
- South African regulations mandate biological monitoring for workers in high-risk industries.
✅ Whole-Blood Cholinesterase is the Most Reliable Marker of Inhibition
- More sensitive than plasma cholinesterase for detecting chronic exposure.
- Reflects long-term enzyme suppression due to its extended half-life (~120 days).
✅ Plasma Cholinesterase is Primarily Useful in Acute Organophosphate Poisoning
- Has a shorter half-life (<10 days).
- Influenced by multiple factors, making it unreliable for low-grade exposure detection.
📌 Accurate biomonitoring is essential to prevent organophosphate toxicity and ensure worker safety.
Background: The Importance of Cholinesterase Monitoring
✅ Why Monitor Cholinesterase Levels?
- Organophosphate and carbamate pesticides are widely used in developing countries due to cost-effectiveness.
- Despite declining use in developed nations, exposure remains a significant occupational health risk.
- Routine biomonitoring identifies workers at risk and helps implement interventions (e.g., medical treatment, reassignment, improved protective measures).
✅ South African Legal Requirements for Biomonitoring
- Regulated under the “Regulations for Hazardous Chemical Agents 2021” (Occupational Health and Safety Act, Act No. 85 of 1993).
- Employers must monitor and protect workers exposed to cholinesterase inhibitors.
📌 Effective biomonitoring helps prevent acute toxicity and chronic neurological disorders.
Types of Cholinesterase Enzymes
✅ 1. Acetylcholinesterase (AChE) - Also Known As:
- True cholinesterase.
- Red blood cell (RBC) cholinesterase.
✅ Location & Function:
- Found in the central nervous system (CNS), neuromuscular junctions, RBC membranes, and the autonomic nervous system.
- Breaks down acetylcholine (ACh), stopping neurotransmission.
- Long half-life (~120 days).
✅ 2. Butyrylcholinesterase (BChE) - Also Known As:
- Pseudocholinesterase.
- Plasma cholinesterase.
✅ Location & Function:
- Synthesised in the liver and found in plasma.
- Hydrolyses various choline and non-choline esters (e.g., succinylcholine, mivacurium).
- Shorter half-life (<10 days).
📌 AChE is the preferred biomarker for long-term exposure assessment, while BChE is more relevant in acute poisoning cases.
Pathophysiology of Cholinesterase Inhibition
✅ Normal Cholinesterase Function
- Acetylcholine (ACh) transmits nerve signals at synapses.
- Cholinesterase breaks down ACh into choline and acetate, stopping signal transmission.
✅ How Organophosphates & Carbamates Cause Toxicity
- Mimic ACh and bind irreversibly to cholinesterase enzymes.
- Prevents ACh breakdown, causing excessive nerve stimulation.
- Results in continuous muscle contractions, leading to tremors, paralysis, and respiratory failure.
✅ Acute vs. Chronic Exposure Effects
- Acute poisoning: Causes fasciculations, paralysis, and respiratory failure.
- Chronic low-grade exposure: Linked to neurodevelopmental disorders, lower birth weights, and chronic lung diseases.
📌 Cholinesterase inhibition leads to severe neuromuscular dysfunction, highlighting the need for early detection and intervention.
Cholinesterase Levels in Biomonitoring
✅ Challenges in Establishing Reference Intervals
- Cholinesterase levels vary widely among individuals.
- Baseline measurements before exposure are essential for accurate interpretation.
✅ Recommendations for Biomonitoring in Workers
- Baseline Testing:
- At least two pre-exposure tests, taken 3–14 days apart.
- Results should agree within 15–20%.
- Routine Monitoring During Exposure:
- Periodic testing during peak pesticide application seasons.
- Intervention Thresholds:
- A >30% reduction from baseline suggests toxicity and requires intervention.
📌 Personalised reference levels ensure accurate exposure assessments and timely protective measures.
Best Biomarker for Cholinesterase Inhibition Monitoring
✅ Whole-Blood Cholinesterase is the Most Reliable Indicator
- Measures combined RBC and plasma cholinesterase activity.
- More sensitive for chronic exposure due to RBC cholinesterase’s long half-life (~120 days).
- Provides a comprehensive view of enzyme inhibition.
✅ Why Plasma Cholinesterase is Less Reliable for Monitoring
- Shorter half-life (<10 days) makes it unsuitable for long-term exposure tracking.
- Affected by liver disease, malnutrition, and pregnancy, leading to false readings.
- Still useful for diagnosing acute organophosphate poisoning.
📌 Whole-blood cholinesterase testing is now the preferred method at Ampath for accurate biomonitoring.
Cholinesterase Testing at Ampath
✅ Test Details:
- Sample Type: Whole blood (EDTA tube).
- Testing Method: Colorimetric reaction following manual haemolysis.
- Laboratory: Conducted at Ampath’s National Reference Laboratory (NRL) in the Specialised Chemistry Department.
📌 Ampath offers advanced whole-blood cholinesterase testing for accurate biomonitoring and regulatory compliance.
Key Takeaways for Clinicians & Occupational Health Practitioners
✅ Cholinesterase monitoring is essential for workers exposed to organophosphate and carbamate pesticides.
✅ Whole-blood cholinesterase is the superior biomarker for long-term exposure detection.
✅ Baseline testing is critical to establish accurate reference values.
✅ A >30% drop from baseline may indicate significant exposure and necessitate intervention.
✅ Plasma cholinesterase is useful in acute poisoning cases but is not reliable for chronic exposure assessment.
✅ Ampath provides in-house, high-quality whole-blood cholinesterase testing.
📌 Regular cholinesterase testing ensures workplace safety and compliance with South African health regulations.